Could this be why your bags didn't make that flight?

A Chicago resident alleges that United Airlines improperly favors cargo over passenger luggage when necessary to keep planes at a safe weight.

Money is the reason, as cargo customers get a refund for bumped shipments while passenger baggage fees are “purportedly nonrefundable,” according to her lawsuit.

"United knows that the removal of cargo from the aircraft's cargo compartment would result in reduced profits, because United receives substantially more money relative to its transportation of cargo than it receives relative to its transportation of checked baggage," according to a complaint filed in Cook County Circuit Court by Gina Spadoni.

The complaint seeks class-action status, saying the number of people affected “likely consists of thousands of individuals.” (Read the complaint below.)

Ms. Spadoni paid $25 to check a bag before boarding a September flight from Chicago to Los Angeles, according to her complaint. But when she landed, she discovered her bag was still in Chicago waiting to be put on another plane, the complaint says.

She alleges that United chose to leave her luggage off the flight because of weight regulations and prioritized more profitable cargo instead. She did not receive a baggage fee refund, per United policy.

'EXERCISES THIS DISCRETION UNREASONABLY'

In its carriage contract, the terms and conditions passengers agree to when they buy a ticket, Chicago-based United does not say cargo will take priority over passenger luggage, said Ms. Spadoni's lead lawyer, Tom Zimmerman of Zimmerman Law Offices PC in Chicago.

United's policy is to carry checked baggage on the same aircraft as the passenger "unless such carriage is deemed impractical." The ambiguous policy allows United to favor cargo and leave behind passengers' checked baggage, he said.

“This is a business decision — United gets to keep all the fees, ship the cargo and still get money from the passengers whose luggage was delayed. United exercises this discretion unreasonably, at no cost to the company and full loss to the passenger,” Mr. Zimmerman said.

“United's removal of checked baggage, or failure to place checked baggage onto the aircraft altogether, is done arbitrarily, capriciously, with improper motive, and in a manner inconsistent with the reasonable expectations of its passengers,” the complaint says.

'WITHOUT MERIT,' UNITED SAYS

United, a subsidiary of United Continental Holdings Inc., said in a statement, "We believe this lawsuit is without merit, and we will vigorously defend against it."

The lawsuit seeks to recover all fees for delayed luggage and ensure that United refunds fees for delayed luggage in the future. It has not received class-action status.

United isn't the only airline that doesn't refund luggage fees if bags are delayed to a later flight, said consumer advocate Christopher Elliott, who publishes on elliott.org. “Everything about air travel is unfair, not just United. Everyone faces different fees, prices and treatments. Airlines make billions from luggage fees and they'll do whatever they can to protect that,” Mr. Elliott said.

The federal Bureau of Transportation Statistics ranked United second, after Delta, in baggage fee revenue last year.

Terry Trippler, an airline industry consumer advocate and blogger, said while class-action lawsuits can help raise awareness about the issue, a more effective strategy may be consumer choice: If United mistreats you, don't fly United.

“I'm a free-market person and . . . what United is doing is their choice. They will continue to pull this off until consumers stop giving them business. Then the company will finally get the message,” Mr. Trippler said.